Quality factors of synthetic quartz glass substrates include the size and density of defects on substrates, flatness, surface roughness, photochemical stability of material, and surface chemical stability. Of these, the quality relating to defects on substrates encounters increasingly rigorous requirements in accordance with the trend of IC technology toward finer feature size processing.
As advances are made on the manufacture of synthetic quartz glass substrates, their surface roughness and defect quality have been improved every year. The traditional process generally includes a polishing step using a cerium oxide base slurry. JP-A S64-40267 proposes the steps of polishing a glass substrate with a cerium oxide base slurry and then precision polishing it with colloidal silica for thereby reducing surface roughness. Currently the precision polishing using colloidal silica becomes the mainstream.
The polishing step is generally followed by a cleaning step. The cleaning step for removing the abrasives and contaminants from the glass substrate surface is divided into several modes, a first mode of showering the glass substrate, a second mode of scrubbing the glass substrate using sponge or the like, and a third mode of immersing the glass substrate in an alkaline or acidic chemical liquid. Further a combination of these modes is widely employed. It is also commonly employed to apply ultrasonic wave during the cleaning step for enhancing the cleaning efficiency.
Recently, polishing and cleaning methods capable of making the glass substrate surface cleaner were proposed. For example, recognizing that once a surfactant is leached out of a polishing pad during polishing, it helps colloidal silica adhere to the substrate, JP 4283061 proposes that polishing is effected with the polishing pad from which the surfactant has been removed. JP-A 2002-131889 proposes cleaning with hydrofluoric acid having an ability to remove colloidal silica. JP-A 2007-284341 proposes cleaning a glass substrate with a cleaning fluid having an etching action on the glass substrate and a higher etching action on metal impurities in a polishing slurry.
JP-A 2004-098278 describes to eliminate raised microscopic defects by polishing with high purity colloidal silica near a neutral level. Such colloidal silica in the neutral range, even in the case of high purity products having a fully low concentration of impurities such as metals, is substantially impossible to continue a steady service because it tends to gel, thicken, or undergo a variation in the particle size distribution of abrasive grains after iteration of polishing steps. Although it might be possible to use colloidal silica in a one-way flow-away manner such that once contacted with the glass substrate, the slurry is discarded, this is undesired from the aspects of economy and environment because expensive high-purity colloidal silica must be used in a large amount.